1. Field of the Invention
Embodiments of the invention described herein relate to antibodies directed to the antigen parathyroid hormone (PTH) and uses of such antibodies. In particular, in accordance with embodiments of the invention described herein, there are provided fully human monoclonal antibodies (mAbs) directed to the antigen PTH. Nucleotide sequences encoding, and amino acid sequences comprising, heavy and light chain immunoglobulin molecules, particularly sequences corresponding to contiguous heavy and light chain sequences spanning the framework regions and/or complementarity determining regions (CDRs), specifically from FR1 through FR4 or CDR1 through CDR3, are provided. The antibodies of the invention find use as diagnostics and as treatments for diseases associated with the overproduction of PTH.
2. Description of the Related Art
Parathyroid glands are part of the endocrine system and produce parathyroid hormone (PTH). PTH regulates the levels of calcium, phosphorus, and magnesium, in the bloodstream, maintaining an appropriate balance of these substances, which is essential for normal bone mineralization. Chronic, excessive production of PTH is known as hyperparathyroidism (HPT). Overproduction of parathyroid hormone leads to an elevated blood calcium level and decreased blood phosphate level. Calcium is removed from bones and calcium absorption from the gastrointestinal (GI) tract increases. The kidneys attempt to compensate for the increased blood calcium level by secreting excess calcium in the urine, which can result in the formation of kidney stones. The effects of increased PTH levels are seen not only in the kidneys, but also in the skeleton, stomach and intestines, the nervous system, and the muscles (R. S. Cotran et al., eds., Robbins Pathologic Basis of Disease 124-647 (4th ed., W.B. Saunders Co., Philadelphia 1989).
In primary hyperparathyroidism, the increased secretion of PTH occurs because of the presence of a tumor, a parathyroid adenoma (˜80%), or less commonly by hyperplasia of the parathyroid (˜15%) or carcinoma (˜5%). As a result of elevated blood calcium levels, symptoms can include kidney stones, bone pain, fatigue, anorexia, nausea and vomiting (L. M. Tierney, Jr., et al., eds., Current Medical Diagnosis and Treatment 1001-02 (35th ed., Appleton & Lange, Stamford, Conn. 1996)). Current medical management of primary HPT is not satisfactory because presently, there are no agents that can produce sustained blockage of PTH release by the parathyroid glands. Surgical removal of part or all of the parathyroid glands is the preferred treatment, although complications such as damage to the laryngeal nerve and prolonged hypocalcemia can occur postoperatively.
In secondary hyperparathyroidism, the excess production of PTH is normally a result of either vitamin D deficiencies (rickets and osteomalacia) or chronic renal failure (CRF). When secondary HPT is due to renal failure, the pathology is characterized by hypocalcemia and hyperphosphatemia and a relative inability to respond to PTH. This resistance to PTH function leads to hyperplasia of the parathyroid glands and excessive production of PTH as the glands try to re-establish normocalcemia and normophosphatemia. The resistance to PTH levels is due to a failure to produce calcitriol (active form of vitamin D) in the kidneys and a failure to excrete phosphate through the kidneys. Calcitriol acts directly on parathyroid glands to inhibit PTH production and the GI tract to promote calcium absorption. Therefore the loss of calcitriol leads to increased serum PTH levels. High phosphate levels also act directly on parathyroid tissue to induce the expression of PTH and can interact directly with calcium to maintain hypocalcemia. The loss of these negative feedback mechanisms account for most of the resistance to PTH seen in CRF (Fauci, A. S. et al., eds., Harrison's Principles of Internal Medicine 2214-47 (14th ed., McGraw-Hill Co. 1998)). In severe secondary HPT, extremely high levels of PTH overwhelm the bone's resistance to the hormone resulting in high serum calcium and phosphate levels that may cause diffuse calcification in the skin, soft tissues, and arteries (calciphylaxis). Such calcification can result in painful ischemic necrosis of the skin and gangrene, cardiac arrhythmias, and pulmonary failure (Tierney et al., supra at 1003).
Currently, secondary HPT is treated medically with phosphate binders such as calcium carbonate and with supraphysiological levels of vitamin D analogues such as calcitriol and doxercalciferol. Not all patients respond to calcitriol and hypercalcemia is a common complication of treatment (Felsenfeld, A. J., J. Am. Soc. Nephrology 8(6):993-1004 (1997)). Calcimimetics, designed as allosteric modulators of the calcium receptor, are also in clinical development as a possible therapy.
PTH is an 84-amino-acid peptide secreted from the parathyroid glands. Its amino acid sequence (Keutman, H. T. et al., Biochemistry 17:5723-29 (1978)) and the nucleotide sequence of the related gene (Hendy et al., Proc. Natl. Acad. Sci. USA 78:7365-69 (1981)) are known. PTH acts through the PTH/parathyroid-related protein (PTHrP) receptor to promote bone resorption and decrease calcium excretion. Human parathyroid hormone (hPTH) circulates as substantially intact hPTH1-84 and fragments thereof. Full length hPTH1-84 and fragment hPTH1-34 are believed to be biologically active, while fragment hPTH35-84 is believed to be inactive. Fragments lacking the N-terminus of PTH (hPTH7-84 or hPTH7-34) are not only inactive, but can also inhibit biologically active PTH in vivo (Horiuchi et al., Science 220:1053-55 (1983)).
Lindall, in U.S. Pat. No. 4,341,755, describes an antibody radioimmunoassay of PTH in mammalian serum. A chicken antibody with a high affinity for the 65-84 portion of human PTH was utilized in the assay. Adermann et al., in U.S. Pat. No. 6,030,790, disclose polyclonal antibodies made by injecting unspecified animals with various fragments of hPTH1-37, which are useful in assays of biologically active PTH. Japan Tobacco, Inc., in Japanese Patent Application No. JP 98337263, filed Nov. 27, 1998, discloses human monoclonal antibodies with reactivity to human PTH or its fragments. However, the disclosed antibodies do not appear to be therapeutically useful, likely due to the fact that the affinities of such antibodies to human PTH are insufficient.
The obstacles to developing a monoclonal or polyclonal antibody to PTH for therapeutic applications have been described and have been attributed to inadequate affinity and immunogenicity (Bradwell, A. R. et al., Lancet 353:370-73 (1999)). Bradwell et al. successfully immunized a patient suffering from parathyroid carcinoma with PTH and the patient produced autoantibodies against PTH. However, due to the clinical need to titrate PTH to an individual target range in the hyperparathyroid patient population, a clinical immunization approach would not be generally applicable given the heterogeneity of immune responses and the need to break tolerance to a self antigen. Thus, the unmet need for a therapeutically useful anti-PTH antibody remains.